1. Field of the Invention
The present invention relates to a construction of a pressure belt for use with an extended nip press in a paper making machine and a method for manufacturing the same.
2. Description of the Prior Art
A conventional press section in a paper making machine has been designed so that a web of paper to be dewatered is carried on a felt and is passed through between a pair of upper and lower adjacent press rolls to squeeze water by linear pressure of the press rolls. Recently, an extended nip press has been developed in which a shoe whose upper surface is somewhat depressed is provided in place of the lower press roll. In such an apparatus, an arcuate surface of the shoe comes into surface contact with the press roll so as to embrace the latter, an endless pressure belt driven by movement of the felt is disposed on the underside of the felt with a web of paper carried thereon and travelling therebetween, and the pressure belt is raised by the pressure shoe to press the felt towards the press roll to thereby form a wide nip region and to increase water-squeezing effect by the surface pressure between the press roll and the pressure shoe. As the pressure belt for the surface pressure nip press (so called an "extended nip press") of the type as described, there is a proposal (GB 2106555A) wherein a layer of synthetic resin such as polyurethane, polypropylene or the like is provided on one surface of a base fabric formed of a synthetic fiber filament. In the prior art, one surface in which is formed such synthetic resin layer serves as a pressure shoe contact surface, and the other surface in which is exposed the base fabric serves as a felt contact surface. However, in said extended nip press, pressure is applied under the condition wherein the shoe is stationary, and therefore, the pressure belt travelling while contacting the shoe cannot travel smoothly unless a friction resistance between the shoe and the surface of the synthetic resin layer is small. To minimize the friction therebetween, lubricating oil is fed between the shoe and the surface of the synthetic resin layer. However, since high pressure is applied to the lubricating oil, oil exudes onto the surface through extremely fine air bubbles such as pin holes present in the synthetic resin layer of the pressure belt and sometimes adheres to the web of paper through the felt to produce unacceptable paper. In addition, in the belt in which the synthetic resin layer is formed on only one surface, both width ends of the belt become warped due to a difference in thermal shrinkage between the base fabric and the synthetic resin when the synthetic resin is cured. When such a belt is mounted on an extended nip press in use, tension of the belt has to be increased so as to minimize the warp of both ends of the belt, causing the undesirable results in that (i) the friction between the belt and the shoe increases to shorten the belt life, (ii) the belt becomes difficult to be driven by movement of the felt, and the exposed surface of the base fabric becomes worn, resulting in breaking of the belt and shortening of the life thereof, and (iii) water removed from the felt cannot be discharged smoothly causing its water-squeezing properties to deteriorate. Of course, the warp of the belt occurs only in both width ends, and since the width of the web of paper to be dewatered is smaller than that of the belt, the belt can be made to travel without increasing tension thereof. However, warped end portions cause lubricating oil to be scraped insufficiently by a doctor, and consequently, troubles such as transferring, scattering or adherence of lubricating oil to the felt and paper likely occur. Further, the other surface in which base fabric is exposed also poses disadvantages in that (i) the base fabric becomes worn severely because of the frictional resistance between the belt and the felt, which is required to be enough great to drive the pressure belt smoothly, and (ii) dewatering efficiency is made deteriorated because of an increasing moisutre content of the felt, caused from a small capacity of the belt surface for receiving the water transferred from the felt.